Sunday, June 15, 2008

Let there be -- hydrogen

If God did create the world by a word, the word would have been hydrogen," said the astronomer Harlow Shapley.

It was Shapley who discovered the shape and extent of the Milky Way Galaxy. "Universe Thousand Times Bigger, Harvard Astronomer Discovers," read the headline in the Boston Sunday Advertiser on May 29, 1921. And that turned out to be just the tip of the cosmic iceberg. Within a decade of Shapley's discovery, astronomers recognized that the Milky Way was just one galaxy among billions.

And what are all those galaxies made of? Hydrogen, mostly.

In fact, the elemental universe consists today of about 90 percent hydrogen, 9 percent helium, and smidgens of everything else. And that's pretty close to the way it's always been, except for the smidgens, which have not been around since the beginning (they were cooked up in stars and splattered into space when stars blew up). When God spoke his mythic creative word, it was exclusively hydrogen and helium that appeared out of the primal fire.

Which comes as something of a surprise to citizens of Earth. After all, hydrogen is not terribly conspicuous in the terrestrial environment. The Earth is mostly composed of stuff you can stub your toe against -- silicon, iron, and other heavy elements. Our bodies are only about one-tenth hydrogen by weight.

In other words, the Earth is a very untypical place by composition, a little oasis of the universe's smidgens of heavy stuff, God's afterthought. The Sun and Jupiter are more typical of the universe, big balls of mostly hydrogen and helium.

Of course, the Sun and Jupiter are not nearly as interesting as Earth, which is why physicists know more about the Sun and Jupiter than the planet that is our home. As everybody's favorite physicist Richard Feynman said: "There's a reason physicists are so successful with what they do, and that is they study the hydrogen atom and the helium ion and then they stop."

For the Shapley and Feynman quotes I am indebted to John Rigden's book about hydrogen, published by Harvard University Press. Rigden is a historian of physics and a biographer of the physicist Isidore Rabi. What he has done in his hydrogen book is show that almost everything we know about atomic physics has come from studying the hydrogen atom.

The hydrogen atom is wonderfully simple -- a single proton and electron in sweet embrace. Every one of the 92 elements when heated in the gaseous state emits characteristic colors of light. And for every element but one the colors stagger across the spectrum in what would appear to be rowdy disarray. The colors emitted by hydrogen line up in orderly fashion like soldiers on parade. Any fool can look at the hydrogen spectrum and know something simple is going on.

Of course, there's simple and there's simple. I have a physicist colleague who routinely tells his students, "If it's not simple, it's not physics," then he goes on to dish out quantum mechanics. Most people think of quantum mechnaics as hard, but the whole theory was teased out of the hydrogen spectrum. "Let there be light," the Creator is supposed to have said. It was hydrogen light that he caused to happen, and he might as well have said, "Let there be physics."

Physicists love to spin out theories of the beginning. They can tell you what happened in the first microsecond of the universe's history, the first second, the first hour, the first million years. They can do it with some confidence even though it happened 14 billion years ago because the universe started simple. First came pure energy. Then quarks. Then protons and electrons. Then hydrogen and helium.

The universe's first stars were made entirely of hydrogen and helium. More to the point, the planets of those first stars were more like Jupiter than Earth, other balls of hydrogen and helium. A thimbleful of Jupiter's interior contains in excess of 10 million billion billion atoms of hydrogen. A pinch of the creation's original simplicity.

The physicist Victor Weisskopf is supposed to have said, "To understand hydrogen is to understand all of physics." He was pretty much right. And maybe, just maybe, to understand physics is to understand everything. That may be true in principle, but never in practice. As Feynman suggested, physics pretty much loses its way once you have atoms with more than one electron.

I love the elegant simplicities of physics, but I wouldn't want to live in a universe that physics could explicitly comprehend. It would be a universe without carbon, nitrogen, oxygen and iron. Without anchovy pizza and cold beer. Without love and lust, birth and death.

"Let there be hydrogen," the Creator said. But it's clear that from the beginning that he had something more interesting in mind.

Further Reading

John Rigden, Hydrogen: The Essential Element.

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